-- Capture each chapter's position in the source site's chapter list so
-- the user-facing list can preserve site order: variants of the same
-- chapter number (e.g. "Ch.14 : PH" next to "Ch.14 : Official") stay
-- adjacent, and non-numeric entries like "notice. : Officials" land
-- where the site placed them rather than clustering at the top under
-- number = 0.
--
-- Lower source_index = closer to the top of the source DOM = newer
-- chapter on this site (it renders newest-first). The list query
-- reverses this with ORDER BY source_index DESC so the oldest chapter
-- appears first in our UI.
--
-- NULL is the sentinel for user-uploaded chapters (no source row) and
-- for crawled rows that pre-date this migration. The list query keeps
-- the existing (number, created_at) tiebreak via NULLS LAST so those
-- fall through to the prior behaviour until the next crawler tick
-- populates the column.
ALTER TABLE chapters ADD COLUMN source_index INTEGER;